You won't get a better deal by buying discounted PHP tickets and then "upgrading" to UPH tickets. I think you must be assuming that you will get the "gate price" of the PHP tickets when you "trade them in". That's not the case. Disney will know exactly what you paid for the tickets - the info is encoded onto the magnetic strip on the back of the ticket. If you purchase a PHP ticket from Disney and decide to upgrade, you will get a credit equal to the price you paid (if the ticket was not used).
I'm not 100% sure what the credit formula is when you trade in tickets that are not purchased directly from Disney. You may only get a credit for what the ticket broker paid to Disney (which is less than the price you paid).
My advice is to buy the tickets you actually intend to use - it'll be a lot less hassle. Enjoy your trip!